What methods can be used to degrade a primary liner?

There are two options that can be used to remove the primary liner.

The first method is to use the Removed option in the Define Composite Properties dialog. This allows you to remove any liner from a composite system. The only problem with this is that the Removed option is relative to the install stage. You need multiple composite liners if you're degrading multiple liners (if you have multiple excavation sequences). The attached model, "Two Tunnel Lining Design Composite Removal.fez", removes the primary liner in the last stage. Note the increase in axial force in the secondary liner as a result.

Tunnel lining design: Removed option used

The second method uses stage properties. In this case use plastic liners and set the strength to zero in the stage when the liner is removed. If you want to keep the liners elastic until the removal stage, simply define very large values of compressive strength and tensile strength. See the attached model, "Two Tunnel Lining Design Composite Strength Reduced.fez", for an example of how to do this. The results are basically the same as the removal file.